Discontinuous transmission scheme

Discontinuous transmission scheme

A base station transmits signals on a frequency (f1) in an active transmission interval (TI) having a starting time and being part of a transmission cycle (TC) including other active transmission intervals for other frequencies (f0, f1, f2, f3) appearing in a predetermined discontinuous transmission scheme that specifies the relationship (Δ1, Az, A3) between the starting times of the active transmission intervals of the frequencies of the set. The mobile station scans the set of frequencies during a scanning cycle (SC) corresponding to the transmission cycle (TC) and determines the frequencies at which the base stations transmit. The start of each scanning is made at a point in time that is specified for the corresponding frequency by the discontinuous transmission scheme and the scanning of one frequency involves ending scanning of this frequency at the end of a time range (TR) of pre-determined length if no signal is detected within this time range.Related Terms:Base Station

TECHNICAL FIELD

The invention relates to the provision of a discontinuous transmission scheme in a radio communication network and how such a scheme is to be handed by a mobile terminal. More particularly, the invention relates to a method for finding base stations, a mobile station capable of finding base stations and a computer program product for making a mobile station find base stations. The invention also relates to a method for transmitting signals from a base station, a base station comprising at least one radio communication unit configured to transmit such signals as well as a computer program product for making a base station transmit signals.

BACKGROUND

Traditionally base stations or cells in wireless wide area networks, such as cellular networks, have been transmitting continuously on some carriers, for instance on known broadcast carriers, i.e. on known and dedicated carrier frequencies.

In this way mobile stations have been able to determine which base stations or cells are in their neighbourhood through merely scanning all possible carriers and frequencies, detect data identifying the base station or cell in question and then connect to the network via the most suitable base station.

However, in future networks, such as LTE (Long Term Evolution) it is of interest for the base stations to be silent, e.g. for the purpose of reducing energy consumption as well as inter-cell interference, in a cell for long durations of time, typically much longer than the ordinary frame lehgth used for ordinary communication. This also makes it harder for mobile stations to determine what frequencies and carriers are provided in their vicinity such carriers and consequently also to connect to a radio communication network.

There is therefore a need for improvement in this field of technology.

The present invention is directed towards improving on this situation, where base stations may be silent during long periods of time, e.g. are operating in a low activity mode.

SUMMARY

One aspect of the present invention is directed towards speeding up the identification in a mobile station of the presence of radio frequencies on which base stations transmit, when the surrounding network is operating in a silent or low activity mode.

This object is according to a first aspect of the invention achieved through a method, in a mobile station, for finding base stations in a radio communication network. The method comprises the steps: scanning a set of frequencies assigned for transmission by base stations in active transmission intervals of a transmission cycle, where the set of frequencies are associated with a pre-determined discontinuous transmission scheme specifying the relationship between the starting times of the active transmission intervals of the frequencies in the set and scanning is performed during a scanning cycle corresponding to the transmission cycle, and

determining the frequencies at which the base stations transmit.

The start of each scanning is made at a point in time that is specified for the corresponding frequency by the discontinuous transmission scheme and the scanning of one frequency involves ending scanning of this frequency at the end of a time range of pre-determined length corresponding to the length of the active transmission interval if no signal is detected within this time range.

This object is according to a second aspect of the invention achieved through a mobile station capable of finding base stations in a radio communication network, where the mobile station comprises at least one radio communication unit that is configured to

scan a set of frequencies assigned for transmission by base stations in active transmission intervals of a transmission cycle, where the set of frequencies is associated with a pre-determined discontinuous transmission scheme specifying the relationship between the starting times of the active transmission intervals of the frequencies in the set, and the scanning is performed during a scanning cycle corresponding to the transmission cycle, and
determine the frequencies at which the base stations transmit.

The start of each scanning is made at a point in time that is specified for the corresponding frequency by the discontinuous transmission scheme and the scanning of one frequency involves ending scanning of this frequency at the end of a time range of pre-determined length corresponding to the length of the active transmission interval if no signal is detected within this time range.

Here the radio communication unit may be equipped with a scanning control unit for performing the scanning and determining the frequencies.

This object is according to a third aspect of the invention achieved through a computer program product for making a mobile station find base stations in a radio communication network,

which computer program product comprises computer program code that when run in the mobile station, causes the mobile station to:
scan a set of frequencies assigned for transmission by base stations in active transmission intervals of a transmission cycle, which set of frequencies is associated with a pre-determined discontinuous transmission scheme specifying the relationship between the starting times of the active transmission intervals of the frequencies in the set, where the scanning is performed during a scanning cycle corresponding to the transmission cycle, and
determine the frequencies at which the base stations transmit.

The start of each scanning is made at a point in time that is specified for the corresponding frequency by the discontinuous transmission scheme and the scanning of one frequency involves ending scanning of this frequency at the end of a time range of pre-determined length corresponding to the length of the active transmission interval if no signal is detected within this time range.

It should here be realized that the start of a scanning can involve the mobile station waiting until the active transmission interval of a frequency it desires to scan is due to be transmitted. It can also involve the mobile station scanning the frequency that is currently being transmitted at a current point in time or is to be transmitted next in relation to the current point in time.

In one variation of the invention, the starting times specified by the discontinuous transmission scheme depend on the corresponding frequencies.

According to one variation of the invention the method comprises the further step of estimating the degree of correspondence of the local timing used by the mobile station and the network timing used in the radio communication network and the step of scanning is performed if the degree of correspondence is found to be sufficient.

According to the same variation of the invention, the radio communication unit of the mobile station may be further configured to estimate the degree of correspondence of the local timing used by the mobile station and the network timing used in the radio communication network and perform the scanning if the degree of correspondence is found to be sufficient. The estimation of the degree of correspondence may be performed by a timing correspondence determining module of the radio communication unit.

According to the same variation of the invention the computer program code of the computer program product may also be further configured to estimate the degree of correspondence of the local timing used by the mobile station and the network timing used in the radio communication network and perform the scanning if the degree of correspondence is found to be sufficient.

It is here possible that the degree of correspondence is estimated to be sufficient if the timing of the mobile station is estimated to differ from the timing of the network by half of the active transmission interval length or less.

According to a further variation of the invention, the method further comprises the step of adjusting the timing of the mobile station based on an estimated difference between the local and network timing if the degree of correspondence is found to be insufficient.

According to this variation, the radio communication unit of the mobile station may be further configured to adjust the timing of the mobile station based on an estimated difference between the local and network timing if the degree of correspondence is found to be insufficient. This may also be performed by a timing correspondence determining module of the radio communication unit.

According to the same variation the computer program code of the computer program product may also be further configured to adjust the timing of the mobile station based on an estimated difference between the local and network timing if the degree of correspondence is found to be insufficient.

Here the scanning may be performed after the local timing has been adjusted.

According to yet another variation of the invention, the method comprises the further step of performing a pre-scanning of one frequency in the set until a signal is received or a scanning time out period is reached corresponding to the transmission cycle and performing the step of scanning on all the frequencies of the set if a signal is received on the one frequency.

According to this variation, the radio communication unit of the mobile station may be further configured to perform a pre-scanning of one frequency in the set until a signal is received or a scanning time out period is reached corresponding to the transmission cycle and perform the scanning on all the frequencies of the set if a signal is received on the one frequency. Here the pre-scanning may be performed by a pre-scanning control module.

According to the same variation the computer program code of the computer program product may also be configured to perform a pre-scanning of one frequency in the set until a signal is received or a scanning time out period is reached corresponding to the transmission cycle and perform the scanning on all the frequencies of the set if a signal is received on the one frequency.

In case a scanning time out period is reached, the performing of the pre-scanning can be repeated for another frequency in the set.

The frequency transmission scheme may also correspond to a frequency reception scheme used by the mobile station in a discontinuous reception mode.

Another object of the present invention is to enable a mobile station to more easily identify the presence of radio frequencies on which base stations transmit when the surrounding network is operating in a silent mode.

This is according to fourth aspect of the present invention solved through a method for transmitting signals from a base station in relation to at least one cell handled by the base station, where the method comprises the step of transmitting signals on at least one frequency, where the transmission on a frequency is made in an active transmission interval having a starting time and being part of a transmission cycle including other active transmission intervals for other frequencies. The at least one frequency is included in a set of frequencies appearing in a pre-determined discontinuous transmission scheme, which specifies the relationship between the starting times of the active transmission intervals of the frequencies of the set and where the starting time of the transmission on the at least one frequency has the relationship to the starting times of the other frequencies of the set defined for this frequency by the discontinuous transmission scheme.

This is according to a fifth aspect of the present invention also achieved through a base station comprising at least one radio communication unit, which is configured to transmit signals on at least one frequency, where the transmission on a frequency is made in an active transmission interval having a starting time and being part of a transmission cycle including other active transmission intervals for other frequencies. The at least one frequency is included in a set of frequencies appearing in a pre-determined discontinuous transmission scheme, which specifies the relationship between the starting times of the active transmission intervals of the frequencies of the set and where the starting time of the transmission on the at least one frequency has the relationship to the starting times of the other frequencies of the set defined for this frequency by the discontinuous transmission scheme.

The transmission may with advantage be provided using a transmission control module of the radio communication unit.

This is also according to a sixth aspect of the invention achieved through a computer program product for making a base station that handles at least one cell transmit signals, where the computer program product includes computer program code which when run in the base station causes the base station to transmit signals on at least one frequency, where the transmission on a frequency is made in an active transmission interval having a starting time and being part of a transmission cycle including other active transmission intervals for other frequencies. The at least one frequency is included in a set of frequencies appearing in a pre-determined discontinuous transmission scheme, which specifies the relationship between the starting times of the active transmission intervals of the frequencies of the set and where the starting time of the transmission on the at least one frequency has the relationship to the starting times of the other frequencies of the set defined for this frequency by the discontinuous transmission scheme

According to one variation of the invention, the method may comprise the further step of entering an energy savings mode in relation to a cell of the network if there are no mobile stations requiring data traffic via the cell, where the transmission in the active transmission interval is made in this energy savings mode.

According to this variation the radio communication unit of the base station may be further configured to enter an energy savings mode in relation to a cell of the network if there are no mobile stations requiring data traffic via the cell, where the transmission in the active transmission interval is made in this energy savings mode.

According to this variation the computer program code of the computer program may be further configured to enter an energy savings mode in relation to a cell of the network if there are no mobile stations requiring data traffic via the cell, where the transmission in the active transmission interval is made in this energy savings mode.

According to another variation of the present invention, the method may comprise the further step of exiting the energy savings mode in relation to a cell if there is at least one mobile station requiring data traffic via the cell.

According to this variation the radio communication unit of the base station may be further configured to exit the energy savings mode in relation to a cell if there is at least one mobile station requiring data traffic via the cell.

The determination of if an energy savings mode is to be left or entered may be performed by an energy saving determining module of the radio communication unit.

According to this variation the computer program code of the computer program may be further configured to exit the energy savings mode in relation to a cell if there is at least one mobile station requiring data traffic via said cell

The invention has many advantages. It allows a fast cell search to be performed in a mobile station when base stations are transmitting in a discontinuous transmission mode. This speed is achieved even if the mobile station cannot receive all frequencies specified in the discontinuous transmissions scheme. This can furthermore be done without the mobile station having to be exactly aligned in time with the transmissions of the base stations or requiring the base stations to resume normal operation. There may thus exist a timing difference between mobile station and base stations. In this way it is also possible to save energy and reduce the mean time between repair of a base station.

It should be emphasized that the term “comprises/comprising” when used in this specification is taken to specify the presence of stated features, integers, steps or components, but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail in relation to the enclosed drawings, in which:

FIG. 1 schematically shows a radio communication network including a number of base stations transmitting signals on a number of carriers and a mobile station that is within range of these carriers,

FIG. 2 schematically shows the timing of a transmission cycle including a set of frequencies according to a discontinuous transmission scheme provided for the radio communication network,

FIG. 3 shows a simplified block schematic of one base station,

FIG. 4 shows a flow chart of a number of method steps being performed in a method being performed in the base station of FIG. 3,

FIG. 5 schematically shows the timing of some frequencies in the set of the discontinuous transmission scheme together with scanning of all the frequencies of the set by the mobile station,

FIG. 6 shows a simplified block schematic of the mobile station,

FIG. 7 shows a number of method steps in a method for finding radio frequencies according to an embodiment of the invention being performed by the mobile station, and

FIG. 8 schematically shows a computer program product according to an embodiment of the invention in the form of a CD ROM disc on which a computer program realizing the methods of the invention is stored.

DETAILED DESCRIPTION

In the following description, for purposes of explanation and not limitation, specific details are set forth such as particular architectures, interfaces, techniques, etc. in order to provide a thorough understanding of the invention. However, it will be apparent to those skilled in the art that the invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the description of the invention with unnecessary detail.

Cellular systems of today often employ a radio communication network, like an access network, for providing base stations handling cells to which mobile stations are able to connect. In these networks it is nowadays also of interest for the cells or base stations to have longer periods of time when they are not transmitting, i.e. when they are silent.

This silence may for instance be kept when there are no mobile stations requiring data traffic, which data traffic may include messaging, multimedia sessions and voice and video sessions. Silence may also be kept in order to save energy. Another reason why silence may be of interest is because a radio circuit can be used a longer time without need to repair. The MTBF (Mean Time Between Failure) may thus be increased. This time of silence discussed here is typically longer than the radio frame used for transmission, i.e. longer than the format of normal radio transmissions.

However in order for a mobile station, often denoted user equipment (UE), to be able to actually find these cells or base stations, there has to be some logic in the way transmissions are made.

Because of this there is a need for a silent or low-activity mode, where the base stations are turned off for a fairly long period of time and turned on for a shorter time. This means that a base station may transmit for a cell in an active transmission period, where normal synchronisation signals and broadcast channels are provided, which is followed by an idle period where there are no transmissions.

This type of silent mode may therefore stipulate that the base station is to refrain from performing downlink transmissions in a cell if there are no or very few active mobile terminals, except for intermittent transmissions of the signals necessary for active terminals in the proximity of the base station to find the idle cell. This is especially of interest where macro cells overlay micro cells. In case of low load, idle terminals can camp on the macro cells and hence use their synchronisation symbols to be synchronized with the network.

As a radio communication network knows to which cells active terminals are connected, it is then straightforward to identity “empty” or very low load cells. Once a terminal moves into the area covered by the idle cell, the base stations needs to wake up and resume normal transmission as well as reception of signals. In order to determine if the base station should resume normal behaviour it may be necessary to detect if a terminal is moving into the idle cell. Such a terminal expects a certain signal, like synchronization signals, reference signals and broadcast channels, in order to be able to find the cell. Therefore despite being in low activity mode, the signals necessary for mobility are intermittently transmitted. The potential energy reduction is here approximately proportional to Ta/(Tp+Ta), where Ta is the duration of the active period and Tp is the duration of the idle period.

The value of Ta should be selected large enough for allowing the mobile terminal to find synchronisation signals with sufficiently high probability of success as well as for being able to perform signal measurements on the cell. The time needed for this depends on the signal-to-noise ratio at the terminal, but if Ta is in the order of 100 ms to 1 s, the probability of being unable to find the idle cell can be expected to be sufficiently low.

The value of Tp should be large enough to allow for a significant reduction in energy consumption. At the same time a too large Tp means that terminals may not find the idle cell. A typical value of Tp may be in the order to 1-10 s.

In order to enable long DTX also for macro cells, it may be necessary to ensure that mobile stations are able to perform initial cell search and mobility measurements on cells in low activity mode. Initial cell search may here be performed through extending the original cell search procedure of the mobile station, where first normal cell search is performed, in which case the mobile station is only able to detect cells in normal operating mode. If this procedure fails, the mobile station may perform extended cell search in order to find cells in low activity mode. Since the base station is only performing intermittent transmissions, the mobile station will need to measure longer on each candidate frequency in order to detect these frequencies.

This extended cell search may in some cases lead to an exceptionally long cell search compared to a normal cell search.

In order to simplify for a mobile station to be able to locate base stations in this scenario, embodiments of the present invention proposes a discontinuous transmission scheme that may be followed by the base stations. With knowledge about this scheme a mobile station can then be able to locate the base stations relatively fast even though they are silent for long periods of time.

One measure that may be implemented to quicken up such cell search is therefore to provide a transmission sequence in which the cells are transmitting, which sequence simplifies the issue of identifying the cells if the mobile station knows this sequence.

Such a situation is schematically disclosed by FIGS. 1 and 2,

FIG. 1 schematically shows a number of base stations 12, 14 and 16 in a part of a wireless network. This part is here an access network or a radio communication network N. There is here a reference base station BS012, a first further base station BS114 and a second further base station BS216. Each base station furthermore transmits data on at least one carrier at a certain radio frequency, where such data may be synchronisation data and broadcast data. A base station may cover more than one cell and it may therefore transmit on more than one carrier, where each such carrier may be assigned to a different cell. However in order to simplify the description of embodiments of the present invention each base station in this example only covers one cell and therefore transmits data on one carrier at one frequency. The base stations are thus transmitting on carriers. These transmissions are furthermore synchronised with a discontinuous transmission (DTX) scheme. The base stations are thus synchronising their transmissions with the discontinuous transmission scheme.

The reference base station 12 here transmits at a reference carrier C0 having a reference frequency f0, while the first further base station 14 transmits on a first further carrier C1 at a first further frequency f1. The second additional base station 16 transmits on a third further carrier C3 at a third further frequency f3. These carriers are all transmitted according to the above-described discontinuous transmission scheme, which will be described later. However, in order to be able to follow this scheme they are all connected to a network clock N_CL 18.

A mobile station 10 is here within the range of these base stations and would be able to receive data on a carrier if tuned to the frequency of that carrier.

How a discontinuous transmission scheme may be provided will now be described with reference to FIG. 2, which schematically shows the timing of a transmission cycle including a group of frequencies according to the discontinuous transmission scheme.

As indicated above the base stations may enter an energy saving mode in which they transmit data more infrequently, i.e. be silent for a long period of time, which may be done because the base station enters an energy savings mode. In order for a mobile station, like the mobile station 10, to be able to detect these infrequent transmissions they are made according to the exemplifying discontinuous transmission scheme in FIG. 2. They are therefore transmitting in a long DTX mode or low activity mode, which mode may be entered when there are no mobile stations in the neighbourhood of the base stations requiring their assistance.

The way transmissions could be structured in this DTX mode is schematically shown in FIG. 2. The frequencies f0, f1, f2 and f3 of four carriers are here shown. These frequencies are frequencies in a set of frequencies appearing in the pre-determined discontinuous transmission scheme. Of these one is the reference frequency f0 and two others are the first and third further frequencies f1 and f3 of the first and second further base stations 14 and 16. In this scheme there is also a second further frequency f2 associated with yet another base station. These frequencies are according to the discontinuous transmission scheme transmitted in consecutive transmission cycles TC. There is here a first transmission cycle stretching between an exemplifying time of zero to a first second, a second transmissions cycle from the first second to a second second and a third transmission cycle from the second second to a third second. As an example each cycle may thus be one second long. As mentioned earlier, the transmission cycle is here much longer than the ordinary frame length used in ordinary network transmissions and then especially in ordinary broadcast channel transmissions.

The discontinuous transmission scheme here specifies a transmission sequence for the carriers. This means that the transmissions at the various radio frequencies follow each other in a certain order, which order may be specified in a standard. The transmission on a frequency is furthermore performed in a limited active transmission interval TI of a transmission cycle TC. This interval TI may be equal in size for all the different frequencies. However, it should be realized that this is no requirement. The active transmission intervals TI may thus differ from each other. Each active transmission interval of a frequency is followed by a silence interval SI, where these two intervals together have the same length as the transmission cycle TC. One active transmission interval TI and one silence interval SI for a certain frequency is thus one transmission cycle long. As can be seen in FIG. 2, the beginning of an active transmission interval need furthermore not follow immediately after the end of a previous active transmission interval. There may thus be silence between active transmission intervals. It is also possible that the active transmission intervals of different frequencies are provided directly after each other or that they may overlap each other.

As can furthermore be seen in FIG. 2, the transmissions on the frequencies are staggered in time. The sequence of frequencies according to the discontinuous transmissions scheme is here exemplified as beginning with the reference frequency f0, followed by the first further frequency f1. Then there is the second further frequency f2 and finally the third further frequency f3.

According to the embodiments of the invention, not only the sequence or order in which the frequencies appear in the scheme is known. Also the time relationship between the active transmission intervals is known.

There is here a first relationship in the form of a first time difference Δ1 between the starting time of the active transmission interval in which the reference frequency f0 is transmitted and the starting time of the active transmission interval of the first further frequency f1. There is also a second relationship in the form of a second difference Δ2 between the starting time of the active transmission interval of the reference frequency f0 and the starting time of the active transmission interval of the second further frequency f2. There is finally a third relationship in the form of a third difference Δ3 between the starting time of the active transmission interval of the reference frequency f0 and the starting time of the active transmission interval of the third further frequency f3. The starting time of the transmission at each frequency is therefore provided at a time defined by the discontinuous transmission scheme. The transmissions scheme may thus specify the relationship between the starting times of the active transmission intervals. In some variations of the invention the time of transmission at a frequency can depend on the actual frequency. This means that the length of the time difference can in fact correspond to the frequency or the difference between the two frequencies, the reference frequency and the further frequency. It can thus be a function of the carrier frequency. The time difference can here be provided for frequencies separated by 200 kHz from each other. In this way it is possible for other entities, like mobile stations, to know of the time differences merely by knowing the frequencies used for transmitting and perhaps knowing which frequency is the reference frequency.

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